1. Field of the Invention
The present invention relates to a method for manufacturing a light-emitting device using an evaporation method.
2. Description of the Related Art
Light-emitting elements, which use organic compounds as a light emitter and are characterized by the thinness, lightweight, fast response, and direct current low voltage driving, are expected to be applied to next-generation flat panel displays. In particular, display devices in which light-emitting elements are arranged in matrix have been considered to be superior to conventional liquid crystal display devices in terms of a wide viewing angle and excellent visibility.
It is considered that a light emission mechanism of a light-emitting element is as follows: when voltage is applied between a pair of electrodes which interpose an EL layer, electrons injected from a cathode and holes injected from an anode are recombined in the light emission center of the EL layer to form a molecular exciton, and when the molecular exciton relaxes to the ground state, energy is released to emit light. As the excited state, a singlet excited state and a triplet excited state are known, and it is considered that light emission can be obtained through either of the excited states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like, in addition to the light-emitting layer.
In addition, an EL material for forming an EL layer is broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a film of a low molecular material is often formed by an evaporation method and a film of a high molecular material is often formed by an inkjet method or the like.
An evaporation apparatus which is used in an evaporation method has a substrate holder to which a substrate is mounted; a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material; a heater for heating the EL material in the crucible; and a shutter for preventing the EL material from being scattered during sublimation. The EL material which is heated by the heater is sublimated and deposited onto the substrate.
Note that in order to achieve uniform deposition, actually, a deposition target substrate needs to be rotated and the substrate and the crucible need to be separated from each other by at least a certain distance. In addition, when films of different colors are separately formed using a plurality of EL materials through a mask such as a metal mask, it is necessary that the distance between pixels be designed to be large and that the width of a partition (bank) formed of an insulator between pixels be large. Such needs are major problems in advancing high definition (increasing the number of pixels) and miniaturization of pixel pitches along with downsize of a light-emitting device including a light-emitting element.
Therefore, as for flat panel displays, it has been necessary to solve those problems as well as to achieve high productivity and cost reduction in order to achieve higher definition and higher reliability.
Thus, a method for forming an EL layer of a light-emitting element through laser thermal transfer has been proposed (see Reference 1: Japanese Published Patent Application No. 2006-309995). Reference 1 discloses a transfer substrate which has a photothermal conversion layer including a low-reflective layer and a high-reflective layer and also a transfer layer over a supporting substrate. Irradiation of such a transfer substrate with laser light allows the transfer layer to be transferred to an element-forming substrate.